The invention relates to a process for upgrading Fischer-Tropsch products by increasing the yield of lubricating base oil and diesel.
The market for lubricating base oils of high paraffinicity is continuing to grow due to the high viscosity index, oxidation stability, and low volatility relative to viscosity of these molecules. The products produced from the Fischer-Tropsch process contain a high proportion of wax which make them ideal candidates for processing into lubricating base oil stocks. Accordingly, the hydrocarbon products recovered from the Fischer-Tropsch process have been proposed as feedstocks for preparing high quality lubricating base oils. See, for example, U.S. Pat. No. 6,080,301 which describes a premium lubricating base oil having a high non-cyclic isoparaffin content prepared from Fischer-Tropsch waxes by hydroisomerization dewaxing and solvent dewaxing.
The economics of a Fischer-Tropsch complex has in the past only been desirable in isolated areas, however, such a Fischer-Tropsch complex can benefit if the production of high-value products in the product slate, such as lubricating base oil and high quality diesel, can be increased. Lubricating base oils typically will have an initial boiling point above about 315xc2x0 C. (600xc2x0 F.). Using the process described herein the amount of lubricating base oils derived from the Fischer-Tropsch synthesis may be significantly increased. If desired, high quality diesel products also may be prepared from the syncrude recovered from the Fischer-Tropsch process. Fischer-Tropsch derived diesel typically has a very low sulfur and aromatics content and an excellent cetane number. In addition, the process of the present invention makes it possible to produce diesel having low pour and cloud points, which enhance the quality of the product. These qualities make Fischer-Tropsch derived diesel an excellent blending stock for upgrading lower quality petroleum-derived diesel. Accordingly, it is desirable to be able to maximize the yields of such higher value hydrocarbon products which boil within the range of lubricating base oils and diesel. At the same time, it is desirable to minimize the yields of lower value products such as naphtha and C4 minus products. The present invention makes these goals possible.
Fischer-Tropsch wax refers to a high boiling fraction from the Fischer-Tropsch derived syncrude and is most often a solid at room temperature. For the purpose of this disclosure xe2x80x9cFischer-Tropsch waxxe2x80x9d will be contained in the higher boiling portion of the Fischer-Tropsch syncrude. Fischer-Tropsch wax contains at least 10% by weight of C20 and higher hydrocarbonaceous compounds, preferably at least 40% by weight of C20 and higher hydrocarbonaceous compounds, and most preferably at least 70% by weight of C20 and higher hydrocarbonaceous compounds.
All syncrude Fischer-Tropsch products as they are initially recovered from the Fischer-Tropsch reactor contain varying amounts of olefins depending upon the type of Fischer-Tropsch operation employed. In addition, the crude Fischer-Tropsch product also contains a certain amount of oxygenated hydrocarbons, especially alcohols, which may be readily converted to olefins by a dehydration step. These olefins may be oligomerized to yield hydrocarbons having a higher molecular weight than the original feed. Oligomerization also introduces desirable branching into the hydrocarbon molecule which lowers the pour point of the diesel and lubricating base oil products thereby improving the cold flow properties of the product. See for example U.S. Pat. No. 4,417,088. In the present invention most of the alcohols will be included in the condensate fraction recovered from the Fischer-Tropsch unit. As used in this disclosure, the term xe2x80x9cFischer-Tropsch condensatexe2x80x9d refers generally to the C5 plus fraction which has a lower boiling point than the Fischer-Tropsch wax fraction. That is to say, that fraction which is normally liquid at ambient temperature.
As used in this disclosure, the term xe2x80x9cC19 minus Fischer-Tropsch productxe2x80x9d refers to a product recovered from a Fischer-Tropsch reaction zone which is predominantly comprised of hydrocarbons having 19 carbon atoms or less in the molecular backbone. One skilled in the art will recognize that such products may actually contain a significant amount of hydrocarbons containing greater than 19 carbon atoms. In general, what is referred to are those hydrocarbons having a boiling range of diesel and below. In general, for the purposes of this disclosure, diesel is considered as having a upper boiling point of about 700xc2x0 F. (370xc2x0 C.) and an initial boiling point of about 300xc2x0 F. (about 150xc2x0 C.). Diesel may also be referred to as C10 to C19 hydrocarbons. Likewise, Fischer-Tropsch wax preferably is comprised predominantly of xe2x80x9cC20 plus productxe2x80x9d which refers to a product comprising primarily hydrocarbons having 20 or more carbon atoms in the backbone of the molecule and having an initial boiling point at the upper end of the boiling range for diesel, i.e., above about 600xc2x0 F. (315xc2x0 C.). It should be noted that the upper end of the boiling range for diesel and the lower end of the boiling range for Fischer-Tropsch wax have considerable overlap. The term xe2x80x9cnaphthaxe2x80x9d when used in this disclosure refers to a liquid product having between about C5 to about C9 carbon atoms in the backbone and will have a boiling range generally below that of diesel but wherein the upper end of the boiling range will overlap that of the initial boiling point of diesel. The term C10 plus hydrocarbons refers to those hydrocarbons generally boiling above the range of naphtha, i.e., the fractions boiling within the range of diesel and lubricating base oils or above about 150xc2x0 C. Products recovered from the Fischer-Tropsch synthesis which are normally in the gaseous phase at ambient temperature are referred to as C4 minus hydrocarbons in this disclosure. LPG which is primarily a mixture of propane and butane is an example of a C4 minus product. The precise cut-point selected for each of the products in carrying out the distillation operation will be determined by the product specifications and yields desired.
EP patent application 0620264A2 discloses a process for making lubricating base oil from waste plastics by use of thermal cracking. U.S. Pat. No. 6,288,296 also teaches a process for converting polyethylene into high VI lubricating base oil using thermal cracking followed by dimerization and isomerization. However, neither process would be suitable for the processing of Fischer-Tropsch syncrude into lubricating base oils as contemplated herein. U.S. Pat. No. 4,579,986 describes a process in which linear paraffins are thermal cracked to yield olefins. The C10 to C19 olefins are treated with a peroxide to make an intermediate which may be converted into lubricating base oil. EP publication number 0584879A1 teaches the thermal cracking of a hydroprocessed Fischer-Tropsch syncrude to prepare lower olefins.
As used in this disclosure the words xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase xe2x80x9cconsists essentially ofxe2x80x9d or xe2x80x9cconsisting essentially ofxe2x80x9d is intended to mean the exclusion of other elements of any essential significance to the composition. The phrases xe2x80x9cconsisting ofxe2x80x9d or xe2x80x9cconsists ofxe2x80x9d are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
The present invention includes a process for upgrading a Fischer-Tropsch feedstock which comprises (a) recovering from a Fischer-Tropsch reactor a Fischer-Tropsch wax fraction containing paraffins and a Fischer-Tropsch condensate fraction, wherein the Fischer-Tropsch condensate fraction contains alcohols boiling below about 370xc2x0 C.; (b) contacting the Fischer-Tropsch condensate fraction with a dehydration catalyst in a dehydration zone under dehydration conditions pre-selected to convert at least some of the alcohols present in said fraction into olefins and recovering a first intermediate effluent from said dehydration zone; (c) pyrolyzing the Fischer-Tropsch wax fraction in a thermal cracking zone under thermal cracking conditions pre-selected to crack the paraffin molecules in the Fischer-Tropsch wax to form olefins and collecting a second intermediate effluent from the thermal cracking zone; (d) passing the first and second intermediate effluents recovered from steps (b) and (c) to an oligomerization zone containing an oligomerization catalyst under oligomerization conditions to form an oligomerization mixture having a higher molecular weight than either of said first and second intermediate effluent; (e) hydrofinishing the oligomerization mixture in a hydrofinishing zone; and (f) recovering from the hydrofinishing zone a C10 plus hydrocarbon product. Preferably, the Fischer-Tropsch condensate fraction recovered in step (a) will have an olefinicity of at least 20% by weight, more preferably at least 40% by weight and most preferably at least 50% by weight. The term xe2x80x9cparaffinsxe2x80x9d refers to saturated hydrocarbons of the methane series also called in the literature xe2x80x9calkanesxe2x80x9d.
In another embodiment of the invention, at least part of the second intermediate effluent is sent to an isomerization unit. The cut selected to be sent to the isomerization unit will depend upon the desired yields and properties of the final products. For example, the isomerization step may be used to improve the quality of the heavy diesel fraction, i.e., the diesel fraction boiling above about 550xc2x0 F. (about 290xc2x0 C.), by lowering the pour point and cloud point. The premium diesel recovered with this embodiment is a high value product which may be used as a blending stock to upgrade lower quality diesel. Alternatively, the cut may include a C20 plus fraction which can be used to prepare a high quality lubricating base oil.
In another embodiment of the invention at least a part of the oligomerization mixture boiling below 370xc2x0 C. is recycled to the thermal cracking unit. In this embodiment paraffins boiling below the upper boiling range of diesel will pass unchanged through the oligomerization unit, be recovered, generally by means of distillation, from the oligomerization mixture, and recycled to the thermal cracking zone for conversion into olefins. This embodiment is intended to maximize the yield of lubricating base oil.
The present invention is also directed to a process for increasing the yield of olefins from a Fischer-Tropsch plant which comprises (a) contacting syngas with a Fischer-Tropsch catalyst under Fischer-Tropsch reaction conditions pre-selected to yield a Fischer-Tropsch product having not less than 20% by weight olefinicity; (b) recovering from the Fischer-Tropsch product a Fischer-Tropsch wax fraction containing paraffins; (c) raising the temperature the Fischer-Tropsch wax fraction sufficiently to vaporize the fraction; (d) steam cracking the vaporized Fischer-Tropsch wax fraction in a flow through reactor under thermal cracking conditions pre-selected to achieve a cracking conversion of the paraffin molecules in the Fischer-Tropsch wax to form olefins of greater than 30% by weight; and (e) collecting an effluent having increased olefin content from the flow through reactor. In order to maximize the olefins present in the Fischer-Tropsch product, it may be advantageous to use an iron-based catalyst to carry out the Fischer-Tropsch reaction. In addition, the conditions in the flow through reactor are critical to the optimal formation of additional olefins from the paraffins present in the wax fraction. The temperature of the wax fraction must be raised to a temperature sufficient to vaporize most or all of the feed. A desirable option is to bleed any remaining nonvaporized hydrocarbons prior to entering the cracking furnace. Liquid cracking of the wax fraction will lead to the formation of undesired paraffins. However, the temperature should not be so high that the wax is over-cracked which results in the formation of excessive amounts of C4 minus hydrocarbons.